1. Field of the Invention
This invention relates to the examination of a body by means of x-radiation or gamma radiation.
2. Description of the Prior Art
The creation of images of x-ray attention coefficients through objects began with the discovery of x-rays by Roentgen in 1895 and has developed continuously. A major advance was the invention in the early 1970""s, by GN Hounsfielch  G. N. Hounsfield of computerized axial tomography, CAT. (U.S. Pat. Nos. 3,778,614, 4,035,647). Hounsfield showed that the linear attenuation coefficients of individual elements in an object could be reconstructed from the measurements of the intensities of x-ray beams that pass in a plurality of independent paths through the object. Since Hounsfield""s invention, the technology of CAT scanning has developed in a number of ways, but all x-ray methods deduce the density distributions from measurements of the radiation transmitted through the object.
This invention is a method for determining the densities of objects or the distribution of densities interior to an object by measuring the intensities of x-rays that are Compton scattered from the volume elements (voxels) of the object. The term x-rays is used throughout the descriptions since it is anticipated that most applications will use an x-ray beam generated by energetic electrons inn an x-ray tube, but it should be evident that all types of energetic photons can be used, including monoenergetic gamma rays, that satisfy the criterion that the energy of the photons are such that Compton scattering dominates the interactions of the photons in the object. The use of scattered radiation to determine densities is fundamentally different in theoretical underpinning, in methodology and in implementing apparatus from the standard methods that use transmitted x-rays to determine densities of interior voxels.
The incident x-rays are rastered across at least one face of an object. The x-rays that are Compton scattered approximately perpendicular to the beam directions are detected in arrays of collimated detectors each of which is sensitive to radiation scattered from a specific portion of the incident x-ray beam.
The distinctive features of the preferred embodiment of this invention are: 1) The energy of the x-rays is high enough so that the interactions in the object are dominated by the Compton effect. In particular, the energy is high enough so that the photoelectric interaction makes a minor contribution in the analysis but it is not so high that pair production is significant. 2) The incident x-rays are collimated into a beam that is scanned through the object in a series of contiguous, approximately parallel paths; the scanning may be accomplished by moving the beam or the container or a combination of both so that the incident x-ray beam passes through every voxel that the detected scattered radiation passes through. 3) The scattered x-rays are detected by arrays of counters that distinguish x-rays that are scattered approximately perpendicular to the incident radiation. The detectors must be capable of sensing the direction of the scattered radiation. Methods for sensing the direction of an incident x-ray are well known; gamma cameras, for example, do so with collimators and position-sensitive detectors. 4) The volume element resolved by this invention is determined by the cross sectional area of the incident x-ray beam times the spatial resolution along the beam path of the origins of the scattered x-rays. The total number of independent measurements is at least equal to the total volume being examined divided by the volume element of spatial resolution 5. The densities of the voxels is rapidly and accurately determined from the totality of measurements by standard mathematical relaxation methods, without the need for transformation into frequency space or the use of back projection, though both of those techniques can be used.
This invention, which we will refer to as Compton Scatter Tomography or CST, is a new modality for tomography, quite distinct from the conventional method of computerized axial tomography, CAT, in which the linear attenuation coefficients in voxel elements in an object are determined from transmission measurements, or Single Photon Emission Computer Tomography, SPECT, in which the directions of gamma rays emitted from a radioactive source distributed in a body is used to measure the distribution of radioactivity. The invention is described in its application for the inspection of containers for contraband such as explosives. It should be appreciated, however, that this invention may be useful for a broad range applications in which a non-destructive method is needed to determine the density distributions of objects.